This invention relates to object location systems, and particularly to systems having a plurality of access points wherein mobile units communicate with the access points using a wireless data communications protocol, such as IEEE Standard 802.11. The invention particularly relates to systems wherein location of a mobile unit is determined by measuring the signal strength of mobile unit transmissions which are received by access points to estimate the range of a mobile unit from the access point. As used in this application the term “access point” is intended to apply to access points as contemplated by Standard 802.11, or other standards, that interface a computer or a wired network to the wireless medium, and also RF Ports and cell controllers connected thereto, as described in co-pending application Ser. No. 09/528,697, filed Mar. 17, 2000, the specification of which is incorporated herein by reference.
A major variable in determination of range of a mobile unit from an access point is the signal strength of the mobile unit. There can be significant variation in transmitted signal strength of transmitter cards (NIC cards) from different manufacturers and even in cards from the same manufacturer from different production runs. Further the transmitter signal strength may vary over the life of a mobile unit and the life of its rechargeable battery.
Differences in transmitter power for mobile units in a system can be calibrated out by doing a measurement of signal strength as received from a known distance and providing a correction factor that is identified with that particular mobile unit. Such calibration requires special calibration procedures for each mobile unit as it is put into service, and, unless units are recalibrated, cannot account for changes in transmitter power as the mobile unit or its battery ages.
It is therefore an object of the present invention to provide new and improved methods of measuring the transmission power of mobile units on a periodic basis to update the correction value associated with such units in a signal strength location system.